1. Field of Invention
The invention relates to the supply of power to resonant loads with voltages greater than 300 V and at frequencies higher than 1 MHz, and in particular to measurements of the current flowing through such loads.
2. Discussion of the Background
Especially for application to automobile plasma ignition, resonators whose resonant frequency is higher than 1 MHz are arranged at the spark plug and are typically supplied with voltages greater than 300 V and are subjected to a current greater than 5 A. This application necessitates the use of radio-frequency resonators with a high quality factor and a high voltage generator, the operating frequency of which is very close to the resonant frequency of the resonator. The more the difference between the resonant frequency of the resonator and the operating frequency of the generator is reduced, the higher the amplification factor of the resonator (ratio of its output voltage to its input voltage) of the resonator. The higher the quality factor, the closer the operating frequency of the generator must be to its resonant frequency.
Numerous parameters have an impact on the resonant frequency: manufacturing tolerances, temperature in the combustion chamber or in the cooling circuit, or aging drift in the resonator components. The evolution of the resonant frequency is thus difficult to foresee or to control.
Servocontrol techniques allow a supply voltage to be kept at a frequency close to the resonant frequency of the resonator. One technique consists in particular in making current and voltage measurements while supplying power to the resonator. Measurement of the phase angle between the current and voltage of the resonator output allows the servocontrol to be carried out.
As shown in FIG. 1, a known device comprises a cable 1 connecting the generator to a resonator 3. The cable 1 comprises conductors 8 and a shield 2. The resonator 3 comprises an inductor 4, a capacitor 5 and a resistor 6. In order to limit the number of conductors between the resonator and the generator, a current measuring device is positioned at the generator. However, the cable 1 introduces parasitic capacitances, illustrated by the capacitor 7 in the equivalent electrical circuit of FIG. 1. Leakage currents then appear in the cable and strongly modify the waveform of the current measured at the generator output.
As illustrated in FIG. 2, the measurement signal 22 at the generator output is phase-shifted relative to the current 21 at the resonator input 3. In addition, this signal 22 is influenced by high-frequency parasitic resonances due to coupling between the resonator and the parasitic capacitances of the cable 1. The performance of the servocontrol is then strongly reduced. No known solution allows the precision of the measurement signal at the generator to be improved for such frequency values.